1. Field of the Invention
The present invention relates to the configuration of a pressure-contact type semiconductor device to be applied to a flat MOS-FET or the like.
2. Discussion of the Related Art
Pressure-contact type semiconductor devices have been mainly applied as power semiconductor devices having large current capacity, and the configuration thereof has been variously changed depending on the kind of device (MOS-FETs, GTO thyristors, and so on).
Semiconductor devices having a flat pressure-contact structure applied to MOS-FET chips are mounted on a metal substrate with the drain electrode surfaces facing downward, and pressure-contact electrodes for the source and gate are mounted on the metal substrate through an insulating material side-by-side with the chips. The pressure-contact electrodes are wire-bonded to the source and gate terminals of the semiconductor elements.
In the case of forming a module by using such a flat MOS-FET, packaging is performed in a manner so that the metal substrate is mounted on a heat-radiation plate of the module and another external connection plate is made to pressure-contact with the upper surface of each of the pressure-contact electrodes connected to the source and the gate. In the packaged state, the pressing force is not exerted to the semiconductor element chips at all but is exerted only to the pressure-contact electrodes.
In conventional pressure-contact type semiconductor devices, a ceramic plate is used as the insulating material and is soldered with opposite side members like a generally-used semiconductor module. Specifically, opposite surfaces of the ceramic plate are metallized in advance and the metal substrate and the pressure-contact electrode are bonded to each other, for example, with space solder or a soldering sheet interposed therebetween through a reflowing soldering method.
Conventional pressure-contact type semiconductor devices having a ceramic plate interposed between the metal substrate and the pressure-contact electrode and bonded to the opposite side members by soldering have experienced problems. If the soldering is performed in the state where the ceramic plate and the pressure-contact electrode are piled up on the metal substrate, it is very difficult to maintain correct parallelism between the plate surface of the metal substrate and the end surface of the pressure-contact electrode. The difficulty in maintaining parallelism arises because of bending due to strain caused by the thermal expansion difference between metal and ceramic or because of unevenness in the thickness of the soldering layer.
In pressure-contact attachment of the foregoing assembly, problems have resulted if correct parallelism between the metal substrate and the pressure-contact electrode mounted on the former is not held as described above. The problems include:
(1) Since the pressing force is not applied uniformly but partially in the pressure-contact state, scattering occurs locally in electrical contact resistance and thermal resistance, so that the operating characteristics of the semiconductor device becomes unstable; and
(2) Scattering occurs locally also in the pressing force to be applied onto the ceramic plate so that the bending stress acts on the ceramic plate to cause cracks, which may cause serious defects such as reduction in dielectric strength.